This
scheme was first proposed at PIMRC '93 in Yokohama by Linnartz, Yee (U. of California
at Berkeley) and Fettweis (Teknekron, Berkeley, currently at U. of Dresden,
Germany). Independently, Fazal and Papke proposed a similar system. Linnartz
and Yee showed that MC-CDMA signals can also be detected with fairly simple
receiver structures, using an FFT and a variable gain diversity combiner, in
which the gain of each branch is controlled only by the channel attenuation
at that subcarrier. At PIMRC '94 in The Hague, optimum gain control functions
were presented. Results showed that a fully loaded MC-CDMA system, i.e., one
in which the number of users equals the spread factor, can operate in a highly
time dispersive channel with satisfactory bit error rate. These results appeared
in contrast to the behaviour of a fully loaded DS-CDMA link that typically does
not work satisfactorily with large time dispersion.

Since 1993, MC-CDMA rapidly has become a topic of research.
At the keynote address of the ISSSTA conference 1996,
Prof. Hamid Aghvami
predicted that the hottest topic in spread-spectrum,
viz.
multi-carrier cdma,
would attract 80% of the research by 1997. Around 2000,
we see that MC-CDMA has attracted tremendous attention,
with entire conference sessions devoted to this.
Mc-CDMA is praised as a modulation solution that
merges the insights due to Shannon (particularly those
relating to CDMA) with insights due to Fourier
(particularly those explaining why OFDM has
advantages in a dispersive channel).

What is orthogonal MC-CDMA?

There are many equivalent ways to describe MC-CDMA:

MC-CDMA is a form of CDMA or spread spectrum, but we apply the spreading in the
frequency domain (rather than in the time domain as in Direct Sequence CDMA).

MC-CDMA is a form of Direct Sequence CDMA, but after spreading, a Fourier Transform
(FFT) is performed.

MC-CDMA is a form of Orthogonal Frequency Division Multiplexing (OFDM), but we
first apply an orthogonal matrix operation to the user bits. Therefor, MC-CDMA is sometimes
also called "CDMA-OFDM".

MC-CDMA is a form of Direct Sequence CDMA, but our code sequence is the Fourier
Transform of a Walsh Hadamard sequence.

MC-CDMA is a form of frequency diversity. Each bit is transmitted simultaneously (in
parallel) on many different subcarriers. Each subcarrier has a (constant) phase offset. The set of
frequency offsets form a code to distinguish different users.

The MC-CDMA method described here is NOT the same as DS-CDMA using multiple carriers. In the latter system the spread factor per subcarrier can be smaller than with conventional DS-CDMA.
Such a scheme is sometimes called MC-DS-CDMA. This does not use the special OFDM-like waveforms to ensure dense spacing of overlapping, yet orthogonal subcarriers.
MC-DS-CDMA has advantages over DS-CDMA as it is easier to synchronize to this type of signals.

Each bit is transmitted over N different subcarriers. Each subcarrier has its own
phase offset, determined by the spreading code.

MC-Code Division Multiple Access systems allow simultaneous transmission of
several such user signals on the same set of subcarriers. In the downlink multiplexer,
this can be implemented using an Inverse FFT and a Code Matrix.

Figure: FFT implementation of an MC-CDMA base station multiplexer and transmitter.

MC-CDMA as a special case of DS-CDMA

Figure: possible implementation of a Multi-Carrier spread-spectrum transmitter.
Each bit is transmitted over N different subcarriers. Each subcarrier has its own
phase offset, determined by the spreading code.

The above transmitter can also be implemented as a Direct-Sequence CDMA transmitter, i.e.,
one in which the user signal is multiplied by a fast code sequence. However, the new code sequence is
the Discrete Fourier Transform of a binary, say, Walsh Hadamard code sequence, so it has complex values.

Figure: Alternative implementation of a Multi-Carrier spread-spectrum transmitter, using the Direct sequence principle.

Receiver design

Because of delay spread and frequency dispersion due to multipath fading, subcarriers are received with different amplitudes. An importance aspect of the receiver design is how to treat the individual subcarriers, depending on their amplitude ri. Options are

Linear combining, by weighting the ith subcarrier by a factor di according to

Doppler

What are the advantages of MC-CDMA?

Compared to Direct Sequence (DS) CDMA.
DS-CDMA is a method to share spectrum among multiple simultaneous users. Moreover, it can
exploit frequency diversity, using a RAKE receiver. However, in a dispersive multipath channel,
DS-CDMA with a spread factor N can accommodate
N simultaneous users only if highly
complex interference cancellation techniques are used. In practice this is difficult to implement.
MC-CDMA can handle N simultaneous users with
good BER, using standard
receiver techniques.

Compared to
OFDM.
To avoid excessive bit errors on subcarriers that are in a deep fade, OFDM typically applies
coding. Hence, the number of subcarriers needed is larger than the number of bits or symbols
transmitted simultaneously. MC-CDMA replaces this encoder by an NxN matrix operation. Our
initial results reveal an improved BER.